RFID technology employs a radio frequency (“RF”) wireless link and ultra-small embedded computer circuitry. RFID technology allows physical objects to be identified and tracked via these wireless “tags”. It functions like a bar code that communicates to the reader automatically without requiring manual line-of-sight scanning or singulation of the objects. RFID promises to radically transform the retail, pharmaceutical, military, and transportation industries.
Several advantages of RFID technology are summarized in Table 1:
TABLE 1Identification without visual contactAble to read/writeAble to store information in tagInformation can be renewed anytimeUnique item identificationCan withstand harsh environmentReusableHigh Flexibility/Value
As shown in FIG. 1, an RFID system 100 includes a tag 102, a reader 104, and an optional server 106. The tag 102 includes an IC chip and an antenna. The IC chip includes a digital decoder needed to execute the computer commands the tag 102 receives from the tag reader 104. The IC chip also includes a power supply circuit to extract and regulate power from the RF reader; a detector to decode signals from the reader; a transmitter to send data back to the reader; anti-collision protocol circuits; and at least enough EEPROM memory to store its EPC code.
Communication begins with a reader 104 sending out signals to find the tag 102. When the radio wave hits the tag 102 and the tag 102 recognizes the reader's signal, the reader 104 decodes the data programmed into the tag 102. The information is then passed to a server 106 for processing. By tagging a variety of items, information about the nature and location of goods can be known instantly and automatically.
The system uses reflected or “backscattered” radio frequency (RF) waves to transmit information from the tag 102 to the reader 104. Since passive (Class-1 and Class-2) tags get all of their power from the reader signal, the tags are only powered when in the beam of the reader 104.
The Auto ID Center EPC-Compliant tag classes are set forth below:
Class-1                Identity tags (RF user programmable, maximum range 3 m)        Lowest cost (AIDC Targets: 5¢ moving down to 2¢ in trillion-unit/yr volumes)        
Class-2                Memory tags (8 bits to 128 Mbits programmable at maximum 3 m range)        Security & privacy protection        Low cost (AIDC Targets: typically 10¢ at billion-unit volumes)        
Class-3                Battery tags (256 bits to 64 Kb)        Self-Powered Backscatter (internal clock, sensor interface support)        100 meter range        Moderate cost (Targets: $50 currently, $5 in 2 years, 20¢ at billion-unit volumes)        
Class-4                Active tags        Active transmission (permits tag-speaks-first operating modes)        Up to 30,000 meter range        Higher cost (Targets: $10 in 2 years, 30¢ in billion-unit volumes)        
In RFID systems where passive receivers (i.e., Class-1 tags) are able to capture enough energy from the transmitted RF to power the device, no batteries are necessary. In systems where distance prevents powering a device in this manner, an alternative power source must be used. For these “alternate” systems (also known as active or semi-passive), batteries are the most common form of power. This greatly increases read range, and the reliability of tag reads, because the tag doesn't need power from the reader. Class-3 tags only need a 10 mV signal from the reader in comparison to the 500 mV that a Class-1 tag needs to operate. This 2,500:1 reduction in power requirement permits Class-3 tags to operate out to a distance of 100 meters or more compared with a Class-1 range of only about 3 meters.
In the design of RF antennas, it is often desirable to achieve an antenna gain pattern that is independent of orientation in any direction, i.e., fully spherical in all three dimensions. Most single antenna designs suffer from attenuation in at least one direction. This usually results in greater difficulties during installations, and reduced reliability over changing environmental conditions. Some solutions have included using multiple antenna and transceiver hardware systems to more completely cover all orientations of the desired signals. These solutions are more costly, and physically larger, due to the requirement of duplicating the transceiver electronics. Other systems have utilized a switched approach where the antenna with the greatest signal is chosen. This requires complex switching electronics and intelligence to properly select the greatest signal.
Therefore, it would be desirable to create an RF design that exhibits the greatest gain, while maintaining a fully omnidirectional (spherical) pattern. It would also be desirable to do so with the fewest, smallest, lowest cost circuitry.
In conjunction with the desire for orientation-independent functionality, it is also desirable to miniaturize the entire transceiver. However, miniaturization urges physical positioning of all of the electronic components near the antenna. The location of conducting elements within the field of the antenna has heretofore generally resulted in the antenna's characteristics being modified, usually in an undesirable fashion. This has been dealt with previously by simply accepting the degraded performance, or by physically separating the antenna from other conductive elements, resulting in an undesirably larger size.
Ideally, the electronics would be positioned adjacent the antenna such that the antenna acts as a virtual ground plane to replace what would otherwise be a printed circuit board. However, prior art antennas tend to be long, thin, and open. The problem is that because of the inductance, these antennas are unsuitable for use as a ground plane as the voltage potentials are different in different portions of the antenna. Because the antenna inductance level is quite different than the circuit, the electronics will exhibit undesirable behavior. For instance, a carrier at 900 MHz represents a different instantaneous voltage at various points on the antenna, so use of different parts of the antenna as the same ground plane would result in different behavior at different times.
What is therefore needed is a way to reduce physical side of the RF device while maintaining optimal antenna characteristics.